A dacquoise is a French torte consisting of meringue layers and buttercream filling. In our two-layer stack, the coffee-flavored frozen-yogurt filling contrasts nicely with the crisp meringue. Immediately after you assemble the dessert, the meringues are crisp, but after sitting a day or so in the freezer, they soften and become almost chewy.
Cover 2 baking sheets with parchment paper. Draw 2 (9 1/2-inch) circles on paper. Turn paper over; secure with masking tape.
Place egg whites, vanilla, and salt in a large bowl; beat with a mixer at high speed until foamy. Add sugar, 1 tablespoon at a time, beating until stiff peaks form. Divide the egg white mixture evenly between the 2 drawn circles. Spread egg white mixture onto the circles using the back of spoon. Bake at 275° for 1 hour or until dry. Turn oven off; cool meringues in closed oven at least 1 hour. Remove meringues from oven; cool.
To prepare filling, beat yogurt and coffee granules until smooth. Carefully remove the meringues from paper. Invert 1 meringue onto a serving plate. Spread filling evenly over meringue. Top with remaining meringue. Cover and freeze several hours or until firm.
To prepare mocha sauce, combine brewed coffee, cocoa, corn syrup, and cornstarch in a saucepan. Bring to a boil, and cook for 1 minute, stirring constantly. Remove from heat; stir in chocolate chips and vanilla. Remove dacquoise from freezer, and let stand for 5 minutes. Cut dacquoise into wedges with a serrated knife. Drizzle the mocha sauce over each serving.